This invention relates to a flow control or modulating valve commonly used in chemical process plants or paper mills. Specifically, this invention also relates to a further improvement of the Chemically Inert Control Valve now patened under U.S. Pat. No. 4,214,730. The major improvement of my new invention over the previous patent is the increase in available flow capacity by providing a direct "line of sight" valve construction and, in addition, providing a valve design where the sealing mechanism is pressure assisted, that is, the seal tightness is improved with increase in valve differential pressure.
Valves of this type are usually positioned by pneumatic disphragm actuators in response to a positioning signal from a control instrument of conventional art. The control of highly corrosive media used for the control of the PH level for bleaching paper or waste water, for example, poses special problems for the process control engineer, since very few materials will resist the strong chemical attack by those media. Plastic materials, such as Teflon*, show nearly perfect inertness, yet are not capable to serve as pressure vessels, i.e., to withstand the working pressure of the controlled medium nor the mechanical stress imposed by the connected piping. It has, therefore, been customary to clad inert valve parts with metal, i.e., to place all working parts of such a valve in a metal shell, usually Type 316 stainless steel. FNT *Tetrafluoroethylene
Prior art valves of this type have conventional plug and seat rings, both made from rigid Teflon. The conventional shape of such valves poses great difficulties on the metal cover design and the supporting metal cover or shroud is usually composed of several separate and interconnecting parts, in order to be able to properly support and protect the internal or wetted valve portion. Such relatively complicated arrangements are, therefore, neither safe nor economical to produce.
My invention has for an important object to provide for a chemically inert, or Teflon valve which is simple to insert into a one piece shroud or metal cover.
A further object is to provide a flangeless or wafer style valve which is easy to install in a pipe system and without need for additional pipe flange gaskets.
Furthermore, my invention allows the use of a fluid pressure actuated valve seat which is self-adjusting, if slightly worn, without need to remove the valve from service.
Finally, yet another objective of my invention is the ability to provide a valve capable of regulating even very minute amounts of flow without being affected by the limitations posed upon conventional Teflon valve plug-seat combinations, i.e., stickiness caused by thermal expansion or seizing due to interference fit of these identical material parts.